1. Field of the Invention
This invention relates to a method for producing silicone elastomer with a painted surface. More specifically, this invention relates to a method for producing a painted surface on a silicone elastomer obtained from a room temperature vulcanizable (RTV) silicone elastomeric composition.
2. Background Information
Silicone compositions which cure to produce elastomers at room temperature are widely used as sealants and caulking materials because they have a unique property profile for applications, such as in building construction. These properties include excellent weatherability, resistance to heat, maintaining their flexibility at low temperatures, ease of application, and rapid cure in place. However, the structures in which such sealants and caulking materials are used are often coated with decorative and protective coatings, such as with paints, varnishes, lacquers, and shellacs. Silicone elastomers have a disadvantage in that they do not have surfaces which accept such organic based decorative and protective coatings. It is desired to have the exposed surface of the silicone elastomers accept painting, for aesthetic purposes, and also may be applied to provide some protection of the surface from the environment. While some silicone elastomers accept some paints, it is very inconvenient to commercially provide certain silicone elastomeric compositions for painting with certain kinds of paint.
Many persons have advanced approaches to solving the painting problem of silicone elastomers. Metters describes in U.S. Pat. No. 3,542,714, issued Nov. 24, 1970, that by incorporating 0.5 to 10 weight percent rosin into dimethylpolysiloxane rubbers, the resulting rubber could be painted. Matters describes many kinds of dimethylsiloxane elastomers and teaches that the paintability of these elastomers can be improved by adding unmodified rosin, modified rosin, or rosin derivatives. Rosin is obtained from the oleoresin or stump wood of pine trees and the primary ingredient in the rosin is abietic acid. Metters teaches that the rosin has a compatibility such that a film of the rosin is formed on the surface of the elastomer. The rosin is added to the compositions of Matters by mixing it with a solvent such as toluene. Solvent, such as toluene is environmentally unacceptable.
Schulz in U.S. Pat. No. 3,812,164, issued May 21, 1974, in U.S. Pat. No. 3,836,502, issued Sep. 17, 1974, and U.S. Pat. No. 3,836,503, issued Sep. 17, 1974, teaches three specific kinds of compounds which can be used in RTV silicone elastomeric compositions which contain an alkoxy silicon compound as the crosslinker. These specific compounds are silanes of the following formulae ##STR1## wherein R is methyl, ethyl or propyl, a is 0 or 1, and x is 1, 2, 3, or 4. Schulz teaches that these silanes are present in an amount of from 0.5 to 10 parts by weight per 100 parts by weight of a hydroxyl endblocked polydimethylsiloxane and that the combination of these silanes and alkoxy silicon compound (crosslinker) provides from 1 to 15 parts by weight per 100 parts by weight of the hydroxyl endblocked polydimethylsiloxane. Schulz also teaches that for these kinds of compositions the cured products can be painted with both latex based paints and oil based paints and that the elastomers have improved heat stability and improved adhesion to substrates.
Clark et al in U.S. Pat. No. 3,957,714, issued May 18, 1976, teach that silicone rubber compositions which cure to elastomers with paintable surfaces are those which are made from hydroxyl endblocked polydimethylsiloxane, monoalkyltriacetoxysilane, an organotin carboxylate, finely divided agglomerated precipitated acicular calcium carbonate, and organic solvent. Although these composition provide a specific RTV silicone composition which cures to an elastomer with a surface which can be painted with at least one decorative organic based paint, such organic solvent containing compositions are environmentally unacceptable today because they have too high a content of volatile organic material, the organic solvent. Furthermore, the high shrinkage caused by the evaporation of the organic solvent has a negative effect on the sealing capability of these sealants.
Shimizu in U.S. Pat. No. 4,358,558, issued Nov. 9, 1982, teaches that a paintable RTV polyorganosiloxane composition contains a silanol-terminated polydiorganosiloxane, an aminoxy functional organosilicon compound (crosslinker), an inorganic filler, and a monohydric unsaturated alcohol containing a carbon-to-carbon triple bond and a hydroxyl group bonded with a primary or secondary carbon atom. Shimizu teaches that these RTV silicone compositions cure to provide adhesive surfaces to which paint will adhere. The monohydric unsaturated alcohols are environmentally unacceptable volatile organic materials.
Fukayama et al in U.S. Pat. No. 4,447,576, issued May 8, 1984, teach paintable, RTV cured polyorganosiloxane sealants made from a hydroxyl-terminated polyorganosiloxane, an organosilicon compound containing at least two amido groups, aminoxy groups or combinations of these groups (crosslinker), and a substituted alcohol which contains as a substituent, an acyloxy, alkoxycarbonyl, cyano, amino, amido, mercapto, or a combination of a carbon-to-carbon triple bond and an ether-type oxygen atom. Fukayama et al teach that their compositions provide sealants with exposed surfaces with sufficient adhesiveness to be coated with paints.
Although each of the forgoing patents teach various solutions to the paintability problem of silicone rubber, they each are faced with difficulties and are specific to kinds of silicone rubber. Even today, the state of the art is such that silicone rubber is consider to be unpaintable, see U.S. Pat. No. 5,063,270, issued Nov. 1, 1991, to Yukimoto et al; U.S. Pat. No. 4,965,311, issued Oct. 23, 1990, to Hirose et al; U.S. Pat. No. 4,906,707, issued Mar. 6, 1990, to Yukimoto et al; and U.S. Pat. No. 4,902,575, issued Feb. 20, 1990, to Yukimoto et al. Endres et al in U.S. Pat. No. 4,985,476, issued Jan. 15, 1991, teach that the so called neutral based joint sealants, RTV-1K systems cannot be coated with lacquers and that some sealing compounds must be brushed with solvent prior to painting.
It is desirable to provide a method suitable for many kinds of RTV silicone elastomeric compositions that cure to elastomers having surfaces paintable with many kinds of protective coatings and at the same time be environmentally acceptable. Such is the objective of this invention.